Silence compiler "always false comparison" warning

Compiling the bitfield test with gcc -Wextra generates those warnings:

 ../../include/babeltrace/bitfield-internal.h:38:45: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
 #define _bt_is_signed_type(type) ((type) -1 < (type) 0)

This is the intent of the macro. Disable compiler warnings around use of
that macro.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com

diff --git a/lib/bitfield.h b/lib/bitfield.h
index f748696ada911163014ec9cad4f4f0abf33ff6c1..42521f2e3f6741826ea9a74cc5d49a89786559fc 100644 (file)
--- a/lib/bitfield.h
+++ b/lib/bitfield.h
@@ -26,6 +26,26 @@
 #error "bitfield.h requires the compiler representation of signed integers to be two's complement."
 #endif
 
+/*
+ * _bt_is_signed_type() willingly generates comparison of unsigned
+ * expression < 0, which is always false. Silence compiler warnings.
+ */
+#ifdef __GNUC__
+# define _BT_DIAG_PUSH                 _Pragma("GCC diagnostic push")
+# define _BT_DIAG_POP                  _Pragma("GCC diagnostic pop")
+
+# define _BT_DIAG_STRINGIFY_1(x)       #x
+# define _BT_DIAG_STRINGIFY(x)         _BT_DIAG_STRINGIFY_1(x)
+
+# define _BT_DIAG_IGNORE(option)       \
+       _Pragma(_BT_DIAG_STRINGIFY(GCC diagnostic ignored option))
+# define _BT_DIAG_IGNORE_TYPE_LIMITS   _BT_DIAG_IGNORE("-Wtype-limits")
+#else
+# define _BT_DIAG_PUSH
+# define _BT_DIAG_POP
+# define _BT_DIAG_IGNORE
+#endif
+
 #define _bt_is_signed_type(type)       ((type) -1 < (type) 0)
 
 /*
@@ -345,6 +365,7 @@ do {                                                                        \
        unsigned long ts = sizeof(type) * CHAR_BIT; /* type size */     \
        unsigned long start_unit, end_unit, this_unit;                  \
        unsigned long end, cshift; /* cshift is "complement shift" */   \
+       bool is_signed_type;                                            \
                                                                        \
        if (!__length) {                                                \
                *__vptr = 0;                                            \
@@ -356,7 +377,11 @@ do {                                                                       \
        end_unit = (end + (ts - 1)) / ts;                               \
                                                                        \
        this_unit = end_unit - 1;                                       \
-       if (_bt_is_signed_type(__typeof__(__v))                         \
+       _BT_DIAG_PUSH                                                   \
+       _BT_DIAG_IGNORE_TYPE_LIMITS                                     \
+       is_signed_type = _bt_is_signed_type(__typeof__(__v));           \
+       _BT_DIAG_POP                                                    \
+       if (is_signed_type                                              \
            && (__ptr[this_unit] & _bt_lshift((type) 1, (end % ts ? end % ts : ts) - 1))) \
                __v = ~(__typeof__(__v)) 0;                             \
        else                                                            \
@@ -412,6 +437,7 @@ do {                                                                        \
        unsigned long ts = sizeof(type) * CHAR_BIT; /* type size */     \
        unsigned long start_unit, end_unit, this_unit;                  \
        unsigned long end, cshift; /* cshift is "complement shift" */   \
+       bool is_signed_type;                                            \
                                                                        \
        if (!__length) {                                                \
                *__vptr = 0;                                            \
@@ -423,7 +449,11 @@ do {                                                                       \
        end_unit = (end + (ts - 1)) / ts;                               \
                                                                        \
        this_unit = start_unit;                                         \
-       if (_bt_is_signed_type(__typeof__(__v))                         \
+       _BT_DIAG_PUSH                                                   \
+       _BT_DIAG_IGNORE_TYPE_LIMITS                                     \
+       is_signed_type = _bt_is_signed_type(__typeof__(__v));           \
+       _BT_DIAG_POP                                                    \
+       if (is_signed_type                                              \
            && (__ptr[this_unit] & _bt_lshift((type) 1, ts - (__start % ts) - 1))) \
                __v = ~(__typeof__(__v)) 0;                             \
        else                                                            \